Pleurophjllidia loveni (probably the Diphyllidia lineata of Hancock), Genia (Acteonia)
coclcsi, and Cumanotus beaumonti, or giving some details of the dentition and anatomy of
these and a few other species.
As is natural in a supplementary part, the majority of the animals figured are rare,
but this rarity may be the result of accidental circumstances as much as of real scarcity.
Limapontia, Genia, and Alderia escape notice owing to their small size. Doi'is testudinaria
is commonly confounded with I). tuberculata, but is not uncommon on our southern coasts
and probably elsewhere. Pleurophyllidia loveni appears to live in sand at moderate
depths and can as a rule be1 obtained only by dredging. Doris maculata and Grimora
papillata are perhaps southern forms which only occasionally reach our shores. Gama-
notus on the other hand appears to be a northern form which is, nevertheless, established
at Plymouth. I t is probable that it occurs on other parts of the British coast but is
local in its distribution.
The drawings are accompanied by various notes in Hancock’s writing, mostly
disjointed and not prepared for publication. They are cited from time to time in the
pages which follow, but the text of the present part is not otherwise based on them.
To the synopsis of the British fauna and the descriptions of new species I have added
chapters on the distribution, nomenclature, bionomics, classification, and affinities of the
Nudibranchiata. It is remarkable how little of Alder and Hancock’s work has proved
to be incorrect, and later researches have added less than might be expected to their
accounts of such species as they were able to investigate thoroughly. But in the Monograph
(which does not, however, represent all their work) they neglect several families,
and since their time numerous new forms have been discovered which have materially
affected our ideas as to the morphology and classification of the group. I have
endeavoured to indicate the character and extent of these additions to our knowledge, at
least as far as they concern the Nudibranchiata of the northern Atlantic.
I must thank the Council of the Hancock Museum at Newcastle-on-Tyne for their
courtesy in lending me all the drawings, notes, specimens, and microscopic preparations
left by Alder and Hancock.1 The preparations consist chiefly of radulae and have been
used for some of the figures of teeth (Grimora, Hero, Tritonia alba, and Doi'is testudinaria)
included in the present plates.
My best thanks are also due to Professor W. G. McIntosh for the loan of a collection
of St. Andrews Nudibranchs, and to Dr. E. A. Allen, Mr. W. I. Beaumont, Mr. R. L.
Crawshay, Mr. R. Elmhirst, Mr. G-. P. Farran, Mr. R. A. Tod, Mr. E. A. Smith, and Mr.
C. W. Walton for specimens and information. To Mr. T. J. Evans, Lecturer in Zoology
at the University of Sheffield, I am indebted for much assistance and for some of the
figures.
1 These included material from India and Ceylon. The i*esults of my examination of the
Nudibranchs of this region were published in the Proc. Zool. Soc., 1906.
i i .
VARIATION AND DISTRIBUTION.
The Nudibranchiata exhibit great variation within the limits of the recognized
species, and it is probableAha* many of‘these should be reduced to the rank of varieties.
Even the land-slugs, though of sober huo, show that the soft integuments of the
Mollusca are very susceptible of changes in colour. In the Nudibranchiata these changes
are more conspicuous because whole divisions exhibit no|p only a greater range and
brilliancy of coloration, but also an unusual fluidity aud flexibility of outline. Many
genera (especially in the Dorididse Phanefobranchiatse) are provided with appendages
which seem to be less specialized for particular functions, than are, for instanei the limbs
of the Arthropoda. Their precise number or shape is immaterial to the animal. Thus
Petyderg guadrilineata1 has typically two unbranch||i dorsal appendages, one on either side
of the branchiae, but occasionally these appendages are absent and frequently one is large
and the other small. Sometimes they are bifid from the base upwards and appear to be
four; sometimes they are palmate and bear live to eight small branches. Similarly the
branchiae vary from five to: nine» and the processes on the oral veil (consideW: to be
typically four) from four to thirteen. The tubercles on the dorsal surface not Only vary
greatly in number but are sometimes separate and sometimes confluent, forming lines or
ridges. These variations can hardly be regarded ifs monstrosities like a bifid tentacle or
a double eye. They merelyglustrate the tendency to vary when variation Joes not
decrease efficiency. The function of the appendages in Polycem is probably to prefect
the branchiae by making it more difficult for ektepal objects to touch them, and
appendages of different shapes may perform this function equally well. Similarly the
rhinophore sheaths of Lomatwtus genet usually bear papillae, but these may vary not only
in different specimens but even on the two sides of the same animal, to such an extent
that I have seen the left rhinophore sheath with a smooth margin while the right bore five
long processes. Other genera in which the external shape offers maqy modifications are
Bornella, Marionia, Oeratosoma, Mwmira, and Triopa.
The appendages which have definite functions such as the rhinophores and branchiae
have naturally more definite and constant shapesjbut the number is not constant either
in the cerata of J2olids or the branchial plumes of Dorids. These latter also show some
variation, in form. For instance, in Stanradoris and Chromodoris they are typically
pinnate but have a tendency to divide and become bipinnate.
Even the anatomical characters are sometimes variable, especially when there is a
question of chitinous structures which may or may not be developed on soft organs. In
1 See Elmhirst in Annals of Scottish Natural History, October, 1908, pp. 227—280, for some
statistics as to tlie variations of this species.